Active line scan sensors presently utilized in the prior art make imagery of an object by directing a beam of light generated, for example, by a laser, to the object and sensing the reflected energy. The beam of light is made to scan the object periodically, similar to a flying spot scanner. Resolution of the sensor is determined by the size of the illumination spot on the object, the spot being scanned such that each resolution element on the object is illuminated one at a time. The reflection from each resolution element may be individually recorded on film or other media. The active line scan sensors described are primarily operated from an aircraft, the object being imaged being the ground and targets thereon.
The beam of light, which may be generated form a laser, scans the ground in a direction perpendicular to the path of the aircraft. For each sweep of the sensor, a narrow line equal to the width of the resolution of the sensor is scanned on each side of the aircraft. The motion of the aircraft allows successive adjacent lines to be scanned, the sensor viewing each resolution element as it is illuminated by the laser beam. Part of the optical reflected energy is detected by photodetectors in the sensor, the resulting electrical video signals being amplified, processed and used to control an optical film writing system, such as a strip map.
The reflectivity of an object at a remote reference plane may be utilized for identification purposes. However, many times the reflectivity differences between targets and backgrounds are intentionally made small for purposes of camouflage.
It is therefore desirable to develop techniques which separate the differences between objects (targets) and backgrounds on the basis of other criteria. One possible criteria is the vertical geometry of the objects in question. It is expected that man made objects and natural objects have different vertical geometry characteristics. Man made objects tend to exhibit extremes in vertical geometry, i.e., being flat, having sudden changes in vertical height, or pronounced vertical shapes whereas the vertical geometry of natural objects tends to lie between these extremes. This additional information, when correlated with reflectivity information, would provide a procedure whereby objects can be separated from backgrounds.